NMR tomographs for imaging investigation methods comprise as essential components besides the main magnet a gradient coil system, generally consisting of three gradient coils which can, independently of each other, be supplied with currents of different strengths.
In this way, preferably linear, constant magnetic field gradients with adjustable strengths can be added to the homogeneous magnetic field B.sub.z aligned along the z-axis, inside the measuring volume of the apparatus, wherein the direction of one of these gradients (dB.sub.z /dz) in general points along the direction of the homogeneous main field B.sub.z and the directions of the two other gradients (dB.sub.z /dx, dB.sub.z /dy) are orthogonal to the z-gradient and to each other transverse to the direction of the main field B.sub.z.
Whereas in the past tomography systems have essentially been used exclusively for diagnosis, in the future there will be an ever increasing demand for combined systems, inside which therapy measures can immediately be followed and controlled with tomography apparatuses. Numerous therapy measures, e.g. surgical, in particular micro-surgical as well as minimal-invasive operations or radiation treatments require access to the patient to be as unobstructed as possible. However, in conventional NMR systems, this access is blocked, in particular in the transverse direction, inter alia by the gradient coil system.
One approach to solve this problem for partial body tomography systems is described for example, in GB-A-2262808, in which a gradient coil system is described, with a measuring volume which is asymmetric with respect to the z-extension of the tomograph, which is particularly suited for the generation of NMR tomograms of the head region of a patient. Since in this known system the measuring volume is shifted to an axial end of the system by means of a correspondingly asymmetric construction of the gradient coils, the head of the patient examined projects into the tomograph only as far as absolutely necessary. In this way, claustrophobia of the patient can be counteracted. However, the very complicated asymmetric construction of the system and the lack of transverse access possibility are a disadvantage.
The gradient coil system known from WO 91/17454, where the gradient coils as a whole can be shifted inside a C-magnet, too, does not enable transverse access to the measuring volume. In the known assembly, the gradient system must approach the measuring object sidewise, since axial access to the main field magnet is blocked due to the construction.
An NMR tomography system according to U.S. Pat. No. 5,304,933 provides free axial access as well as limited transverse access via small side holes at the circumference of cylindrically shaped coil systems which can be shifted inside each other. A gradient system is provided which can be inserted as a whole in the axial direction into the room temperature bore of a cryomagnet for the generation of a magnetic main field, as well as a radio frequency (RF) coil system which for its part can be inserted axially into the gradient coil system. By means of corresponding small holes at the circumference of the cylindrical coil systems which are located so as to be aligned in the measuring position of both systems in the total assembly, small, narrow instruments can be inserted into this apparatus from a transverse direction into the measuring volume. However, no free transverse access to the measuring volume which could be used by a surgeon is present in this system.
Finally, from U.S. Pat. No. 5,414,360, mentioned above, a gradient coil system for therapy tomographs is known providing transverse as well as an axial access to the measuring volume in such a way that good access from all directions is ensured as required for a treatment of the patient examined. In the known gradient coil system this is achieved by a particular "bent" arrangement of the partial gradient systems which are symmetric with respect to a central plane across the measuring volume perpendicular to the z-axis, wherein each partial coil comprises azimuthal segments having a relatively great axial distance from the central plane and a relatively great radial distance from the z-axis and which are connected to other azimuthal segments by radial conductor segments which are relatively axially close to the central plane and which have a relatively small radial distance from the z-axis. Compared to gradient coil systems which are merely located on a single cylinder surface, such gradient systems have, however, a larger inductance and a lower efficiency, and as a consequence, for identical ampere turns a lower gradient strength inside the measuring volume is reached. Moreover, the shielding effect of the active shielding elements of the gradient coils as provided for the known system to the outside and against interference from the outside is less. Finally, the particular arrangement of the gradient coil segments on different radii requires a relatively complicated winding procedure and a considerably larger radial extension of the completed gradient systems.